Production of superopaque vitreous enamels



Patented Mar. 6, 1945 PRODUCTION OF SUPEROPAQUE VITREOUS ENAMELS Paul 0. Stufit, Baltimore, Md; assignor to Pemco Corporation, a corporationof Maryland No Drawing. Application Jun 21, 1940-,

- Serial No. 341,779

3 Claims: (Cl. 106-48) The. present invention relates to the production of enameling and/or ceramic glaze frits whereby the opacity of the resultant product is materially improved and closely controlled, and

more particularly to the preparation of enameling and/or glaze frits of a type wherein the opacity is produced by crystallites dispersed through the frit by the process of devitriflcation or crystallization. I

In the enameling of sheet iron and steel articles it has long been realized that the enamel coating should be highly resistant to failure from mechanical impact, distortion of the enameled shape and the strains due to assembly, shipping, and daily use. At the same time the opacity or diffuse reflectance of the enameled articles must.

be as high as possible in order to present a pleasing white, appearance. t

In the past it has been exceedingly difflcult to produce porcelain enameled and glazed Ware meeting all of these criteria. If, for example, the coating be applied sufflciently thin to have a 3 maximum resistance to mechanical abuse, the opacity or covering power of the enamel is reduced markedly, making necessary the use of a high percentage of expensive mill-added opacify ing materials. If, on the other hand, the porcelain enamel or glaze coating, even with the high millv opacifier addition, he applied sufliciently heavy to give the desired covering power and whiteness, the thickness of the coating is in-' creased to such an extent as to give rise to a considerable loss due to mechanical failure during the enameling or assembling operations, or in shipping, or in general use. The porcelain enamel smelting operation so that the porcelain enamel glass issuing from the smelter has approximately the same basic opacity or covering power, if drawn directiyfrom the smelter, as the final product after same has been quenched in water. J

The opacity present in enamels produced with this type of opacification is limited by the amount of opacifying material which may be added to the glass without injuring its physical properties.

In other words, the introduction of excessive amounts of opacifying materials may give rise to enamel glass of unpleasant surface texture lacking in gloss and surface hardness, or in enamels of improper composition for application -to sheet steel or iron.

. It has been known for many years that enamels may also be prepared which derive their opacity through the supersaturation of the molten matrix with the opacifying agent, and its subsequent reprecipitation ior crystallization on the cooling of the enamel frit. The preparationof enamels of this type of opacification has been so difflcultthat. their commercial use has not been deemed practical; except in minor cases where expense, of

manufacture was-not a material consideration,

For example, if the reprecipltated crystalloids are too large .or improperly diflused through the enamel, a mottled appearance results which is extremely unpleasing to the eye and results in an unsalable product. If, on the other hand, the

, crystalloids are too small or too few, the opacification power of the enamel -is greatly reduced.

frit in general commercial use today derives its opacity or covering power in two ways; first, through the formation of insoluble crystals due to reaction in the smelting operation; and second, through the addition of so-called mill-added opacifiers. The opacity of a porcelain enamel glass depends upon the diffusion through the glassy matrix of a large number of minute particles. of materials of markedly greater or lesser optical density than said matrix, Morespecifically, the present day commercial enamel depends for its opacity upon the addition of fluoridebearing materials as, for example, cryolite, iiuorspar or sodium silicofluorideflwhich have lower indices of refraction than the glassy matrix which, of course, comprises the-body .of the enamel glass. At the same time, antimony and/or; its compounds, for example, antimony trioxide or sodium antimonate, are added, these being materials oi greater indices of refraction than the glass. During the smelting operation these materials form reaction products insoluble in the enamel glass. The nature of said insoluble products is not definitely known.v However, the

particles form during the-initial (phase of the Enamels of this type of opacification, if taken from the smelter before quenching. produce a practically .clear, transparent glass. However, the water quenched material shows the'development of considerable opacity which is. further augmented during the firing operation oi the enameling procedure. It has beenfound that in order to gain the greatest final opacity or covering power with this type of enamel, the enamel irit or comminuted glass resulting fromthe quenching operation should give the maximum "possible-opacity. Examples of this type of porcelain enamel are found in those which are opaci.

fled by compounds of arsenic and zirconium.

Arsenic opacified enamels have been used for many years in the jewelry industry, but due to the poisonous nature of the major opacify'ing component; that is, the oxides of arsenic, andthe very expensive and complicated manufacturing procedure required to obtain a salable enamel,

their use on sheet'steel has been deemed commercially impractical up to the present time.

The use of zirconium opacifld enamels is also well known, as is the fact that it has been recently possible to produce enamel frit composl tions of superior opacity and physical properties to the fluoride and antimony opacifled enamels,

that is, enamels which maybe applied at the very low application weights required for greatest mechanical strength, while at the same time offering adequate opacity without the use of large amounts of expensive mill added opacifiers. However, up to the present time it has not been possible to produce enamel frits of this type of suflicient uniformity for satisfactory commercial use. Using the same raw batch mixture a series of finished enamels may show a variation of as much as from 5% to 7% in diffuse reflectance, making a large part of the product because of its varying opacification properties, commercially unsalable. This defect has limited the production and use of this'type of zirconium opacified enamel. Furthermore, enamel frits of this type, when milled and applied to sheet steel, show wide variations in opacity with increased firing. For example, the application of a light additional coating of the enamel and subsequent reflring of same in order to "repair a defective or damaged article results-in i: great gain in opacity, giving the so-treated article ,such an increased reflectance onopacityas to make it commercially unsalable. This phenomenon is not encountered with antimony opacifled enamels of the type above referred to wherein any increase of opacity is in direct ratio of thickness of the coating-applied rather than the combination of increased coating and additional firing.

.mal zirconium opaciiied enamels. It has alsobeendiscovered that by controlling the temperature of the quenching medium in which the molten zirconium'opaciiled frit forming material is quenched, said temperature being considerably higher than that normally hitherto used for the quenching of zirconium frits, that the so quenched zirconium frit does not exhibit any wide variation in its retired opacity, or, stated dlfierently maintains its uniform opaclflcation properties upon continued heating or firing and reheating or reflring. conium opacifled enamels which one would ex- It has alsobeen discovered that zirpect would be annealed when employing a temperature limit which inhibits the produc ion of particles of frit of large sire and of such hardness as makes milling of the frit particles exceedingly difllcult or impossible. Usually, said upper limit is generally in the neighborhood of 190 R,

quenching operation is resorted to solely for the purpose of comminuting the hit and so reducing its particle size as to make possible the ready milling of the resulting material in a ball mill to the proper fineness for application to the sheet steel or iron ware. In the production of enamels of this type, it has been found desirable to use as low a quenching temperature as possible, since this produces a finer comminuted frit and also one which is more readily milled.

In carrying out the present invention, the zirconium opacified enamel is preferably treated in accordance with the method and apparatus set forth in Patent No. 2,137,931, granted to the Porcelain Enamel & Manufacturing Company of Baltimore, as assignee of Craven and Knudsen. Reierring to the disclosure of said patent, it is there set forth that the maximum desirable quenching temperature ranges from about to 160 F. Very satisfactory results on ordinary enamels are obtained by using a temperature considerably below 120. If the quenching temperature of an antimony opacified enamel is increased above to F., a partially annealed material is produced which is hard and difiicult to mill, and if the temperature is much above 160, the hardness of the resulting antimony opacified enamel is suchas to make milling thereof almost impossible. Further, using a high temperature of quenching for antimony opacifled enamels resultsin the size of the comminuted particles being greatly increased, thereby increasing the time and the expense of the subsequent milling operation. From the experience with the quenching of-antimonyopacified enamels, one would be led to the conclusion that enamels in general should not be quenched at temperatures above 160 deg. F.

Zirconium opacified enamels, as stated above, depend for a great measure of their opacity upon the recrystallization or devitriflcation 0t zirconium compounds in theglassy mixture during the cooling or quenching operation. If the temperatureof the quenching fluid is maintained in the neighborhood of deg. F. to deg. F.,

there results a'product of uniformly high opacity which is free from all of the defects which have in the past characterized this type of enamel. If the quenching temperature be decreased below 160 deg. the opacity of the resultant enamel frit is decreased from 4%-5%. Further, zircor nium opacified enamel frits, and particularly zirconium opacifi'ed enamel frits of the compositions hereinafter recited, do not exhibit the properties of forming the large particles and the hard and annealed particles when quenched at tempera, tures in excess of 160 deg. F. as do the antimony opacified types of enamels when quenched at similar temperatures; said zirconium opacifled enamel frits quenched at temperatures as high as 200 deg.;F. show no indication of the iormation of large particles and hard and annealed particles, thus indicating that-even higher temor this may be considerably greater, depending upon the character of the molten zirconium opacifled enamel being treated, including its chemical and physical characteristics, and the character and the amount of the zirconium compound carried by the enamel or ceramic glaze or similar material for increasing the opacifying power of the enamel or ceramic glaze.

It is desired to point out that in the manufacture oi fluoride-antimony opaciiied enamels, the

,peratures' may be satisfactory and satisfactorily used where the compositionand the properties of the particularzirconium opacified enamel frit indicates that temperatures higher than 200 deg.

. F. may be required to obtain the development of maximum opacification' properties.

- The present invention will be illustrated by the into asmelter which may be of any convenient type commercially in use, but preferably is of the I continuous type. .Heat is applied and the temperature is held at about between 1950 to 2000 F. or a little higher until the'enamel is smelted to a smooth. thread; that is, is free from insoluble raw material particles. It the smelter be of the so-called batch type, it is desirable in most cases to further .heat treat the molten material for a period of approximately 30 minutes after the melting is complete. In either case,-after the enamel composition has been properly melted, it is discharged from the smelter into a quenching pitcontaining a cooling fluid which is preferably, although not necessarily, water, the temperature of the fluid lafdjacent to th .falling stream of molten mate a1 being maintained at about between 180-'190 F. Preferably, although not necessarily, the molten enamel flows through an inert heating medium adapted to keep the-air from unduly cooling the molten enamel upon its entry into the quenching bath. all as set forth in Craven and Knudsen patent, No. 2,137,931. Preferably, although not necessarily, the molten stream of material passe through a counter flowing stream of quenching medium of the desired temperature, and the quenched material and the other-constituents may illustrativelmbut not by way of limitation, vary as follows:

I Per cent SiO2 30.0 to 40.0 A1203 7.0 to 9.0 B203 12.0 t'13.0 KNaO '10.0 11616.0 CaO 3.5 to 6.0 F2 5.0 to 10.0 ZIO g 12.0 to 15.0

- ZnO L 2.5 to 8.0 P205 0.5 to 3.0 TiOz 0 to 2.0

' The present invention may also be carried out using the raw batch compositions set forth in U. S. Patent No. 1,944,938.

, The advantageous results produced by the ture in accordance with the present invention areindicated by the following:

is continuously removed from the bottom of the I quenching pit, all as set forth in said patent to Craven and Knudsen. It is desirable-that t he temperature of the quenching medium be con- Two batches of a raw enamel mix capable of producing the frit set forth in Example I were smelted under similar conditions. The first batch herein designated .frit A was quenched at a water temperature of 80 F., while the second batch trolled during the quenchingperiod, so that ex cessive water temperatures-are not" produced.

Preferably, the temperature is controlled by the of the same raw composition smelted under exactly similar conditions-was quenched at a temperature of 180 F. to produce the herein designated frit B. The resulting batches of frits A and B were each separately milled with "1% of; ball.

By adding 1% of the usual opacifier which is rm B, 74.0

usually a zirconium type opacifier tothe aforementioned milling mixture, there is obtained 'opacified enamels having the following diffuse re- 1 flectance:

Per cent Frit 1r 74.0

I Frit B 78,0

I Per cent SiOz 32.75 A12O3 8.15 B203 12.40 KNaO. ;j 14.0 I, 09.0 5.7

p.75 ZlOz ,13.7 ZnO I Y 2.75v P205 1.75 lIiO'r 0.00

The following is an additional-example of a frit which has been-produced by the method while the present invention is applicable to. .frit carrying various percentages of zirconium opaciflers, in general the zirconium constituent thereby indicating and demonstrating that. the

opacity variation persists regardless of treatment after the smelting operation. I

A comparison of the opacity of zirconium enamels smelted in accordance with the present inabove set forth. I

Example]! I I I The frit produced had the following calculated chemical composition: I

' Per cent- S120 I 32:5 A1203 8.0 B203 I 12.20 KNaO r I '13."!5 C 3 I 5 3 F: 8.50 ZIOI 13.5 ZnO 2.75 .PzOs t 7 2.25

I Zirconiun iAcompound;

vention and the best available antimony type of opacifiedenamels, when milled with similar opacifler addition; and -with the hitherto mentioned antimony opacified enamel milled with 8% of an antimony opacifler, follows:

I Percent .difluse reflectance at- I Percent opacifler 'iype enamel opaciiled with- Frlt B lintifinony compound..." 70. 4 r

. The data 5.. forth in the above table clearly 40 gItIzS-ISQ. 5 0 gms./sq.

indicate that frit A opacii ied with zirconi- I Further, frit ample I, said zirconium irit 3" having 'been quenched in accordance with the present invention, is far superior in opacification power as. compared to the antimony enamel when milled with similar amounts of mill-added opacifier. B has a somewhat superior. opacification power over the antimony opaclfied enamel'milled with a high percentage of opacifier, namely 8% Stated differently, in accordance with the present invention, it is possible to increase and maintain the uniformity of the 'opacification power or difiuse reflectance of enamel irits and ceramic glazes which derive their opacity by the,.uniiorm distribution throughout the glassy matrix of minute crystallcid particles precipitated from supersaturated solution in the molten glass during the quenching of the same by maintaining the temperature of the quenching medium, which is usually water, above 160, and preferably aif a temperature of above 180, which will inhibit the production of particles of frit of large size and of such hardness'as makes milling or the frit particles exceedingly diilicult or impossible. In general, as stated, said upper limit is in the neighborhood of 190, although this may be somewhat greater, as heretofore pointed out. The preferred temperature ofquenching the smelted material set forth in Example I is 180. When what less than 180 or somewhat greater.

As stated, it is preferred to use the method and apparatus set forth in U.'S Patent No. 2,137,931. However, satisfactory results may be obtained using other methodsof quenching and other apparatus, and it is to be clearly understood that the present invention is not-limited to any particular method of quenching or any particular apparatus, but is capable of being utilized with any of the prior art methods or with the use any prior art quenching apparatus.

- By quenching the zirconium opacifled enamel in the manner set forth, there is produced a. unihitherto, using the prior art temperature of quenching there was avariance of 41:0 7%.

The zirconium opaciiying constituent may be any material which will function to opacity, but preferably isa material containing silica, such as zirconium silicate, ocdouble silicates of zirconium and sodium, orct zirconium and potassium. zirconium oxide may be used .in preference to zirconium silicate Zi-BiOI, but

since the silicate, that is,zircon,i.8 cheoperthan [the oxide, it is preferable to use thelatter M, terial. "These zirconium materials are set forth acro a the composition is slightly diflerent, the temperature of quenching may be difierent, either someand obviously other equivalent zirconium containing compounds may be used.

For a part of the zirconium there may be substituted other materials, such as titanium compounds and the like.

While certain typical enameling compositions containing zirconium opacifier' have been hereinset forth, it is obvious that these are merelyillustrative, and not by way of limitation. In other words, various kinds of vitreous enamels and ceramic glazes containing zirconium-opacifylng agents may be treated in accordancewith the present invention.

The present invention has been illustrated by the quenching .of molten zirconium enamel in water or other aqueous medium. Usingthis quenching medium, it is obvious that the quenching temperature must be kept below the boiling point of water. The molten zirconium opacified enamel may be cooled and comminuted in a nonaqueous medium, as for example a gaseous m'edlum,and in such case the temperature at which the molten enamel is quenched to cool and comminute the same may be greater than 200 F. or 205 F. The limiting factor is that the enamel must not be quenched at a temperature which will anneal the frit particles and make them so hard as to produce difliculty in milling, or make milling impossible and/or produce particles of frit of large size.

The following data are illustrative of the advantages obtained by quenching the zirconium opacified frit at 160 F. or higher. Before the present discovery was made, a 63,000 pound continuous run producing a zirconium type of enamel was made and the resulting enamel exhibited a variation in opacity from 66.3 to 75.9% or about 9% when applied-at 65 grams per sq. it., which is obviously far beyond any reasonable commercial limit. It is this variance in opacity upon firing which has made it substantially impossible prior 'the impracticed eye,

to the present invention to use a zirconium opacified enamel. V

Using the present invention, a comparable enamel was produced in a 78,000 lb. run, and the enamel showed a variation ofonly 73.1 to 15.6% when applied at 50, grams per sq. it, which is a variation of only 2.5%.' The average vari' tion between the batches for'this latter run w very much less than the. maximum. Forexample, the average basic opacity for this run was (4.5%, so that the deviation of maximum'and minimumopacity from the average was in the neighborhood .of 1% to 1.5%, which is well within the limhe of acommercial enamel and not visible to temperatm'e oi! 160 or above is'such that itv is not visible to the impracticed eye of the average was such a wide variation in opacity between successive portions of the enamel produced in a i continuous procem thatjthe average consumer could easily see the variation, and so the enamelwas hot commercial.

,Whatisclnimedis:

opacifled molten' fit-forming materialinto a bath maintained at a temperature between F. and 205' F.,"whereby there is by up! ample and not by m oflimitatlcn, produced: irit characterized by uniform opacity,

Stated functionally, the variation in opacity upon and reflrlng of a zirconium opacifled enamel quenched ,at a.

tween 160 F. and 200 F., whereby there is produced a frit characterized by uniform opacity,

said frit maintaining its uniform opacity and covering power upon firing and refiring.

3. The method of producing opaque zirconium opacifled frit developing its ultimate diffuse reflectance from crystallites dispersed through the kit upon the molten flit-forming material beingquenched comprising flowing the molten zirair from unduly cooling the molten material prior to its introduction into a quenching bath and then introducing the so-treated molten frit-forming material into a quenching bath maintained at a temperature between 160 F. and205 F., said bath temperature producing no substantial annealing of the zirconium opacifled frit particles, and providing a vitreous enamel successive batches of 4 which exhibit substantially uniform difluse re- ;flectance, and which maintain said difluse reflectance upon firing and retiring.

PAUL C. STUFF'I.

conium opacifled frit-torming material through an inert heating medium adapted to prevent the 1 

